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Floating Raft Construction - All about Rafts

Aquaponic Rafts are generally made from construction grade polystyrene sheets. In large commercial aquaponic raft systems, entire uncut 4 x 8 foot sheets are used and the aquaponic raft tanks may be 8 feet wide by 100 feet or more in length. New seedlings are placed in clean empty raft boards at one end of the system and harvest ready boards are removed and emptied of plants at the other.

In home aquaponic systems smaller raft tanks and smaller aquaponic raft boards are used. 2 x 4 feet is a common size raft board as it is easy to handle. The actual size of your raft aquaponic boards will be determined by your actual aquaponic raft tank or DWC. I have a floating raft tank that is 4 x 8 feet on the outside, so after space for the wood framing and vinyl lining the inside dimension is 43" x 90" and my floating raft system boards are cut down accordingly.

Watch for a sale on construction Styrofoam sheets and buy the size you can get inexpensively, then cut it to fit your system. In my case I found a sale on 2 x 8 foot sheets as shown below. The Styrofoam can be from 1-2 inches thick, however 1" Styrofoam is more than adequate and easier to handle, although it can break if not lifted properly when it has a full aquaponic plant load.

Once the Styrofoam is cut to size, then it can be marked for drilling. In home aquaponics where we grow a variety of crops, an 8" spacing is suggested. This is a good general size and you get 18 holes in a 2 x 4 sheet. This spacing will be a little small for large head lettuce (not generally grown), and a little wide for baby leaf lettuce, but is just right for most full size leaf lettuce, salad mixes, Romaine and butter head varieties. It also works well for basil and other herbs.
How you cut your holes in the aquaponic rafts will vary depending on how you start your seedlings. Here are 3 scenarios using different potting methods:

Hydroponic Net Pots - Net Pots come in different sizes, generally 1" up to 4" depending on the type of plants you are growing. In backyard aquaponics you would pick a common size that is big enough for the largest plants you intend to grow. Then you would use a standard hole saw to cut circular holes through the Styrofoam board that is large enough for the pots to slip through but smaller than the lip around the top of the pot, allowing the pots to nest into the aquaponic raft, without falling through.

Horticultural Horticubes - These cubes, as seen in the pictures below, have a large base (1" square) and narrow top. This makes them more difficult to work with as they tend to fall through the aquaponic raft, into the water below. In this case, mark out 1" squares on the foam board and use a sharp, thin bladed knife to cut out the square. Angle the cuts such that at on top the square is 1" wide, but on the bottom it's 3/4" wide. Then the Horticubes will sit in the holes without falling through. I have found that in addition to being more difficult to keep in the holes, these cubes also tend to break apart over time. For this and other reasons I have switched to Rockwool cubes.

Rockwool Cubes - This is my favorite method and widely used in aquaponics. These 1" cubes come with a wide 1" square top that tapers down to a point at the bottom (see pictures below). They are made of long fiber Rockwool, which resists breaking apart and crumbling. In this case, we use a tapered drill bit to cut a tapered hole. I'm using a bit that is 1 1/8" at its widest size. This hole is perfect for these cubes, which sit down inside the hole without falling through, and keeping the aerated water from splashing up. Once you have one sheet marked and drilled, you can use it as a template to mark all the other sheets. When you drill the hole, run the drill at full speed and as the drill reaches its full depth, go slower and allow the drill bit to "polish" or "melt" the foam slightly giving a nice smooth finished hole. Note: some aquaponic system crops have more massive root structures, such as celery, tomatoes or cucumbers. These crops, if raised in a aquaponic raft system, do better in large net pots or with a larger Rockwool or Horticubes block and raft hole. In backyard aquaponics we tend to grow these crops in the aquaponic gravel beds and put the lettuce and herbs in the aquaponic rafts. If you decide to build a mixed system aquaponic raft system, then you may want some aquaponic raft boards holed for different sizes and spacing.

Examples of Rockwool and Horticubes

Once you have your rafts properly marked, cut and/or drilled, then you can place them in your system. Over time the aquaponic rafts in your floating raft system will get slimy and the underside coated with goo. Do NOT clean this off. This coating is made up of the beneficial bacteria that is needed for your system to convert ammonia to nitrites and then to nitrates. Furthermore, many cleaning agents are deadly to fish and/or plants. Your aquaponic raft boards should not be allowed to dry out either. When they are removed from one end of the system, the crop should be harvested immediately and the floating raft boards returned to the other end of the system to be filled with new seedlings. In backyard aquaponics or hobby aquaponics systems you will probably never remove your floating raft boards and will just remove the mature crops from the board as they ready. Here are a couple of pictures of aquaponic rafts in use. Notice the older 2" pink Styrofoam in my system, with square holes and 6" spacing, using Horticubes. It is being replaced with 1" aquaponic rafts at 8" spacing using Rockwool cubes. The color of your floating raft system Styrofoam is not critical, the Styrofoam is the same and does not easily breakdown in the water or sun.

Having used the new cubes and holes for 4 months now, I like the tapered Rockwool cubes. If I could find a tapered drill bit that started at 1 1/2 or even 1 3/4 at the widest, this would allow for a larger hole for the roots and the cubes would sit down in, recessed. Most leaf lettuce does OK with the 1 1/8 bit, but Celery, larger head lettuce, and tomatoes need a larger hole. However, I have searched for a larger tapered bit and have not been able to find one.